1. Field of the Invention
This invention relates in general to a particle separator for fluids, and in particular to a particle separator for fluids using optical pressure.
2. Description of the Related Art
The invention of the laser has made possible many new areas of research and technology. Unique optical properties allowing a laser to be highly focused have made detailed studies of radiation pressure possible. Most important is the laser's ability to focus down to a tiny spot size, resulting in a large photon density. This large number of photons translates into a significant amount of radiation force applied to a particle in the beam path. Radiation pressure has been used to trap and direct particles caught in the focus of a laser beam. Manipulation of the beam focus and beam position can be used to move particles into desired positions and configurations. The types of objects that have been optically trapped include glass and polymer spheres, viruses, bacteria, and biological cells. Recently, size-based separation of particles flowing in a fluid opposite to the direction of laser propagation has been achieved.
In recent years, a technique has been developed, termed optical chromatography, which involves laser separation of differently sized particles in the 1-10 micron range. When particles in a liquid flowing within a capillary encounter a laser beam propagating in the opposite direction, the particles are subjected to optical pressure near the beam focal point (i.e., the region of highest photon density) intense enough to impart momentum sufficient to overcome fluid drag forces. The result is that particles in the fluid become trapped and move against the fluid flow until the beam diverges and the photon density decreases. The particles remain stationary when the optical pressure equals the force exerted on the particles by the liquid flow (i.e., Stoke's force).
For a sphere of refractive index n2 in a medium of lower refractive index, n1, the force due to optical pressure of the laser, Foptical—pressure, is given by equation 1:
                                          F                          optical              ⁢              _              ⁢              pressure                                =                                                    2                ⁢                                                                  ⁢                                  n                  1                                ⁢                P                            c                        ⁢                                          (                                  a                  ω                                )                            2                        ⁢                          Q              *                                      ,                            (        1        )            where P is the power of the laser, c is the speed of light, a is the sphere radius, ω is the beam radius, and Q* is the conversion efficiency of optical radiation pressure to Newtonian force on the particle. The term (n1P/c) defines the incident momentum per second in a medium of refractive index n1. The dimensionless parameter, Q* defines the conversion efficiency of optical pressure transfer arising from light reflection and refraction based upon geometrical considerations and is calculated using the Fresnel reflection and transmission coefficients, which in turn depend upon n2, the refractive index of the particle.
Separation in a liquid flow is measured by the distance particles travel away from the focal point against the fluid flow. This distance traveled is the optical retention distance, z: the point at which the optical pressure equals the force exerted on the spheres by the liquid molecules and is defined, according to Equation 2:
                              z          =                                                    π                ⁢                                                                  ⁢                                  ω                  0                  2                                            λ                        ⁢                                                                                                      n                      1                                        ⁢                    PQa                                                        3                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    η                    ⁢                                                                                  ⁢                    vc                    ⁢                                                                                  ⁢                                          ω                      0                                                                      -                1                                                    ,                            (        2        )            where P is the power of the TEM00 mode laser, c is the speed of light, a is the sphere radius,  is the beam radius at the focal point, λ is the wavelength of light, ν is the velocity of the particle in the water flow, and n1 is the viscosity of water. The refractive index of the particle is used in the calculation of the efficiency of optical pressure transfer, Q.
Optical pressure has been used extensively in research and industry for biological size-based micromanipulation. The chemical effect on optical pressure in bacteria has been observed: small chemical differences in the surface coatings have been shown to result in large force differentials on different strains of the same species of non-pathogenic bacteria. However, the theoretical chemical dependence, development, and use of optical pressure chemical differentials for separation have not yet been demonstrated.